1. Field
The disclosed embodiments relate generally to power supply circuits.
2. Background
FIG. 1 (Prior Art) is a circuit diagram of a conventional power supply circuit 1 that supplies power to an external load 2. Power supply circuit 1 receives power from a battery (not shown) via VBAT voltage supply terminal 2 and ground terminal 3. Power supply circuit 1 outputs a desired output voltage VOUT onto output terminal 4. A bandgap voltage reference 5 outputs a reference voltage VREF such as, for example, 1.2 volts. A resistor divider made up of resistor 6 and resistor 7 divides the voltage VOUT on output node 4 such that when a desired voltage (for example, 4.0 volts) is present on output node 4 then the voltage VREF will be present on node 8. A differential amplifier 9 compares the reference voltage VREF to the voltage on node 8 and drives the voltage on the gate of transistor 10 accordingly. The current flowing from drain to source within transistor 10 is mirrored by transistor 11 and a large pass transistor 12 such that a proportional current flows from the VBAT terminal 2 the through pass transistor 12 to output terminal 4. If the current flowing through pass transistor 12 to output terminal 4 is too small such that the voltage on node 8 is less than reference voltage VREF, then differential amplifier 9 increases the voltage on the gate of transistor 10 such that the current flowing through pass transistor 12 increases until the voltage on node 8 matches the reference voltage VREF. If, on the other hand, the current flowing through pass transistor 12 to output terminal 4 is too large such that the voltage on node 8 is higher than the VREF, then differential amplifier 9 decreases the voltage on the gate of transistor 10 such that the current flowing through pass transistor 12 decreases until the voltage on node 8 matches VREF. The voltage on output terminal 4 is therefore regulated by a voltage control loop.
In some applications, noise may be present on the battery voltage VBAT due to multiple circuits in addition to power supply circuit 1 being coupled to the same battery. If, for example, the battery voltage VBAT were to drop momentarily from the desired 4.0 volt supply voltage, down to 3.0 volts, and then return back up to the desired 4.0 volts, then this momentary drop in VBAT should not be translated into a corresponding momentary change in the supply voltage VOUT supplied onto output terminal 4. A radio frequency (RF) die that has sensitive radio frequency circuitry for a cell phone may, for example, receive power from output terminal 4. The 4.0 volts supplied from output terminal 4 is to remain constant despite momentary fluctuations in battery supply voltage VBAT.
The ability of the power supply circuit to output a constant output voltage VOUT despite a change in its input voltage VBAT is measured by a quantity called power supply rejection ratio or PSRR. The PSRR of a power supply circuit, in units of dB, is determined by dividing the variation seen in the output voltage VOUT by the variation in the input voltage VBAT, and then taking the logarithm of this quotient, and then multiplying the resulting value by 20. In general, the higher the gain of the voltage control loop, the better the PSRR (a better PSRR means that the PSRR number is a larger negative number). The PSRR of the power supply circuit, however, is frequency dependent. The voltage control loop responds well to low frequency changes in the input voltage VBAT. For faster changes in the input voltage VBAT, however, the control loop may be undesirably slow such that VBAT variations are communicated through the power supply circuit and are introduced into the output voltage VOUT. In the cell phone application described above where a sensitive RF die is powered by the power supply circuit, a PSRR rejection of −40 dB or better is desired for input voltage frequency variations from zero Hz up to 100 kHz.
One limitation on the speed of the voltage control loop is the size of pass transistor 12. Pass transistor 12 is generally made to be large so that the power supply circuit 1 can supply the desired amount of supply current to load 2. In one example of the circuit of FIG. 1, pass transistor 12 is made approximately 48 millimeters wide by 0.4 micrometers long (W/L=120,000) so that the power supply circuit can source a needed 300 mA of supply current in a cell phone application. Pass transistor 12 therefore occupies several square millimeters of die space. In addition to occupying an undesirably large amount of die space, the large size of pass transistor 12 in the voltage control loop serves to slow the response of the voltage control loop such that the PSRR of the power supply circuit at 100 kHz is worse than it otherwise could be. An improved power supply circuit is desired.